PRINCETON — The Princeton University football went off at halftime leading 21-20 Saturday afternoon, but it wasn’t a very confident lead.

The nation’s fourth-leading run game had only 67 yards. The nation’s fourth-best run defense had already giving up a 69-yard touchdown run and allowed 136 yards on the ground.

If there was a turning point in Saturday’s 42-26 win over Lafayette, it came at the start of the second half.

Two plays in Lafayette fumbled at midfield, and while Princeton wound up punting, it did attain field position when Lafayette set up at its own eight.

On second down Anthony Gaffney picked off a pass at the 28 and returned the ball to the 20. One minute later Princeton led 28-20.

Lafayette would close to within 28-26, but Princeton’s defense would prove it had an attitude adjustment and Lafayette was finished.

Twenty-eight pass completions by two Princeton quarterbacks and four TD passes by junior Quinn Epperly carried the offense, while the defense finished with a couple of interceptions, a couple of forced fumbles, 10 tackles for loss and four sacks.

“The biggest thing was we weren’t gap-responsible or as physical as we could have been,’’ said strong safety Matt Arends about the early going. He led the team with 10 tackles.

“Once we saw we could take it to ’em, we hit it,’’ he said. “After the first big run it was a little flat. It was a series of things that happened that made it that we were just like, ‘We’re just gonna kick these guys’ butt.’’’

Or, as senior captain and defensive end Caraun Reid put it, “We started to be relentless.’’

That’s all Princeton’s high-scoring offense needed to see.

After an exchange of punts late in the third quarter, Princeton moved 51 yards in eight plays. Three passes went to junior Connor Kelley, one of which was a circus catch that went for 29 yards down to the one. On the next play the score was 35-26.

Kelley, a converted quarterback, finished with eight catches for 102 yards. Senior Roman Wilson had another big day with 10 grabs.

“The way our offense is designed, a lot of guys are contributing,’’ Kelley said. “If the run game is not working we have other options, and this game was evidence of that.’’

After a few punts into the fourth quarter, Princeton put on a 73-yard drive, using 3:15 of clock, to go ahead 42-26.

The Leopards had one final drive that ended on a failed fourth down play, and the Tigers ran out the final 5:05 in front of a crowd of 7,494.

To Lafayette coach Frank Tavani, “It was a tough day out there. They put on a lot more pressure and just out-played us in the second half.’’

Running back Ross Scheuerman, former Allentown High School star, finished with 162 yards on 22 carries and was the first player to rush for three touchdowns against Princeton in three years.

“I guess they schemed it up in the second half and figured out what we were doing,’’ he said about the Princeton defense. “But it comes down to execution.’’

Teammate Justin Adams, former Notre Dame High standout, had three receptions for the Leopards, but was taken out of the game in the second quarter and sent to a local hospital where he underwent a CT scan. He did not return to the stadium, but Tavani felt a concussion was part of his condition.

Several Lafayette players were hurt in the game, but nothing hurt worse than blowing a 20-11 first quarter lead.

Lafayette, now 1-4, plays its second of four straight road games next Saturday at Harvard. Princeton takes its 3-1 to Brown next Saturday evening before returning to New England on the 26th to face the Crimson.